How to Build a Crazy Accurate AR15
Now with over a decade of gun building and gunsmithing, I have learned a lot and definitely tested the limits of my abilities. What shocks me is with every extra morsel of AR-building knowledge gained, I realize how much I still have to learn. At one time I believed that only unobtainium level barrels, triggers and components would deliver amazing accuracy, however I have actually found it is more about how the components go together and sometimes really simple things can make or break accuracy. My previously inexperienced brain from years ago had told me things like receiver truing, barrel torque, headspacing were all witch doctor level blessings, but now I know better.
Truing the Face - Truing is assuring the face of the upper receiver is square and uniform via a lapping process. Similar to blueprinting a Remington 700 bolt gun, this process uses a Brownells or similar drill powered AR Upper Lapping Tool to square up the shoulder of the receiver’s barrel mounting face with an abrasive lapping compound.
This process assures when the barrel is mounted and torqued down, that it mates to a square and uniform receiver face where the barrel nut torque is perfectly distributed around the circumference.
When the barrel heats up, everything will stay aligned instead of a barrel being forced off in the direction of lowest torque point around the barrel extension and stringing shots.
Honestly it is shocking to me how huge a difference this can make and how out-of-true cheap lowers can be and how much even the premium lowers can benefit from this build technique. I have witnessed this simple process take an unhappy friend’s 1.75-inch grouping match grade barrel and deliver sub-MOA accuracy after Truing. I have yet to find a receiver that had a flawless trued receiver face from the factory, so I would say this process could improve any rifle to some degree - usually about 20%, but could be more. Any surface finish including anodizing will have variances in surface thickness and CeraKote is really bad, so ensuring that all uppers are trued before assembly is critical for shot to shot accuracy.
AR Barrel Extension Bedding - AKA Gluing. After the barrel receiver is trued, the best accuracy will be from a perfect rigid fit between the barrel and receiver that is so tight you can hold the receiver and barrel not will slide out even without a barrel nut. With that noted, we all know that usually our AR barrels will just easily slip-in with the greatest of ease which is not the best for accuracy. The optimal fit is demonstrated by JP Rifles who undersize their receivers and require a cool the barrel, heat the receiver slip fit process to install the barrel. Nearly every top premier tier AR builder beds the barrel to the receiver with at least a “gluing” or heat-fit method… so maybe they know something. My experience has shown that it is well worth the extra time.
Even with a sloppy fitting barrel to receiver DIY setup we can replicate this by bedding the barrel to the receiver. The process is simple and requires something like LockTite 638 or 620 Retaining Compound. LockTite 638 is harder and will fill a wider gap range, LockTite 620 has a 50-degree higher temp (450-degree) rating but half the gap fill range. Yes, you can use Purple, Blue or Red LockTite and maybe even wood glue, however 638 Retaining Compound is the right product for the job and designed exactly for this type of metal to metal bedding application - at $14/bottle will last you a lifetime. My preference is 638 since it will fill larger gaps and is a harder compound with higher PSI rating.
My application is to clean and degrease the barrel extension and do the same for the receiver. I do put the world’s lightest coat of WD-40 inside the receiver to assure that if perhaps someday I would want to remove the barrel, that I can without torches and witchcraft. This also makes post install cleanup of excess 638 a bit easier. Ideally the 638/620 will stick to the barrel extension and not well to the receiver. I screw on a muzzle device and tap the muzzle with a rubber mallet, quickly re-clean the barrel nut threads to remove any trace of 638 with rubbing alcohol and torque down the barrel nut with copper anti-seize on the barrel nut threads. The Retaining compound sets in about 4 minutes so you do need to move the process along. Once together, I clean off any excess 638 from the outside and inside of the receiver and then let everything set up for a few hours. At that point you should have a barrel which is essentially mated to the receiver and will improve accuracy.
Proper Barrel Nut Torque - Similar to the accuracy improvements truing the receiver will offer, proper barrel torque can improve accuracy as well. Armorer spec for AR15s barrel torque is between 30-80 ft/lbs of torque with copper anti-seize (no freaking lock-tie) applied to the barrel nut threads. Most premier builders recommend 35 ft/lbs. With that noted, most home builders wildly over-torque their builds. I have witnessed some home builds exceed 120 ft/lbs of torque and require a torch to remove the barrel nut.
The general rule is to use as close to 30’ish ft/lbs of torque as possible and only increase if the gas tube is not indexing correctly on the barrel nut. Most top builders suggest using a pack of inexpensive shims under the barrel nut to align the gas tube indexing as a preferred method instead of increasing torque. No need to monkey hang from your barrel nut wrench.
Proper barrel nut torque reduces stress between the barrel and receiver which has shown to increase accuracy, but more importantly, it prevents the potential of thread locking the barrel nut to the receiver, or worse, shearing the barrel extension pin and/or torque twisting the entire upper receiver out of alignment. There is a school of thought that very high torque should be used in the 70-ft/lb range, however if the receiver is trued, this high torque nets nothing more than just added stress to the barrel receiver union.
The Zen State of Headspacing - Accuracy is More about Headspace than the Barrel - AR15 bolts ARE NOT self-headspacing but they tolerate a lot of slope before functioning is impared. If there were no locking lugs, self-headspacing could be a little true, however the fit of the lugs between the barrel extension locking lugs and then the bolt face headspace to the chamber are all critical to maximizing accuracy. The relationship between the barrel and bolt is really interesting in that the tolerances for the home AR builder can be too minute to measure - sometimes you just need to find a bolt that gets along with your barrel.
Shortly before I started to headspace check every barrel/bolt pair, I had this incredibly budget friendly Anderson build using more or less 100% Anderson components including barrel and bolt. Obviously Anderson is not known for premo accuracy, but the shocking part was it was stupid sub-MOA accuracte due to an unusually tight headspace spec usually reserved for match grade spec rifles. Swapping out to another bolt carrier group would show off some rather unimpressive groups, but with that one bolt, it was spectacularly accurate.
I have always kept that lesson in mind and religiously check headspace with Forrester Go, No-Go, and Field gauges and test different bolts until I can optimize accuracy. When I have horrible accuracy issues with a known quality barrel, 98% of the time I can track it down to some type of headspacing issues or I need to try some different bolts to find one that hits the zen state. Looking back, I have to wonder if some of those premium barrels I could not get to group early in my editorial career that I sold, maybe just needed a different bolt. I usually have a Brownells, Aim Surplus, YM, Sharps, Fail Zero, and WMD bolts to test and it is interesting that sometimes an expensive barrel likes a cheap bolt.
A “GO” gauge checks the minimum chamber dimension and assures you are a “Go”. A “No-Go” gauge checks the maximum SAAMI spec for the chamber - the bolt should not close on the No-Go gauge, if it does the chamber’s headspace is too loose, but can still be fired until it also fails the longer “Field” gauge. If the bolt closes on a Field gauge, you have a substantial issue on your hands and the gun should not be fired.
What I look for is a fairly tight lug lockup which just passes the “Go” gauge. The best accuracy will usually be had when the chamber size is closest to the “Go” gauge minimum size. I have found that a bolt that just barely closes on a “Go” gauge will deliver the best accuracy, but sometimes not the best reliability. The closer that spec is to the No-Go gauge size, the less accuracy you will generally have. Many barrels will act as if they cannot perform well, when in fact with tighter headspacing bolt, they can be spectacular.
Sometimes though the headspacing will check out just fine, but there is some other voodoo at work. As with the Anderson build, I have had numerous other builds which have benefited from just testing various bolts side-by-side until the best accuracy and reliability is achieved. Sometimes too tight of headspacing will impact reliability and too loose impacts accuracy - you have to find the marriage that optimizes both. I have found that buying a factory headspaced bolt from the barrel manufacturer or opting for a factory headspaced barrel/bolt combo will almost always deliver the best possible accuracy.
For DIY builds, I have seen some transformations that can tighten or loosen groups, however recently a bolt swap on a high dollar Ballistic Advantage Hansen profile .223 Wylde 16” barrel while getting sighted in has been my most extreme example. The barrel was not shooting well at all even with a range of match ammo which these barrels have shown to consistently deliver sub-½ MOA accuracy with match ammo. Both the shown groups were shot with American Eagle ammo which has a reputation as neither great nor horrible in the accuracy department. Same, gun with groups shot only about 3 minutes apart only the bolts were changed. It was an astounding transformation from a 4-inch group to a ½-inch group with just swapping between two high dollar Sharps and Y/M National Match bolts which both headspaced within range just fine. Voodoo I tell you.
This wild and extreme improvement was something I had never before with just a bolt swap where all other components were left identical and the two groups were shot minutes apart. I was so shocked I actually repeated the test and it shockingly delivered the same result. My point is that headspacing can make or break the most expensive barrel in the world on AR15s or even the best ammo, so always check your headspace, and perhaps even test through a handful of bolts for the best accuracy possible. That Ballistic Advantage Hansen barrel went from a future as a tent stake to one of my most consistently accurate builds with just a bolt swap. This technique is something you can try with your AR right now swapping other bolts from other builds.
The Zen Voodoo of Ammo Selection - I am a big proponent of barrels that shoot consistently which for me means the zero does not dance around a whole lot and does not deliver wildly different accuracy results regardless of ammo. A good barrel for me shoots decently well with the cheapest mil-spec or white box ammo and really well with any quality and/or match ammo. This is a really hard requirement, but one reason I usually like Faxon, Barnes Precision, Black Hole, and Ballistic Advantage, and if you can find the Feddersen (Tri-Arc branded) barrels. Generally, I have found that polygonal, 4R or 5R type rifled barrels are usually less picky across ammo. Unless I have a barrel with insane accuracy with a particular round that I know I can get or reload, I am not a fan of picky “only one ammo shoots well” barrels.
A good example for me was a gorgeous Proof Research barrel for a Ruger RPR which delivered insane single hole accuracy with one or two specific rounds and was completely inconsistent for me with everything else. The guy that bought it loves it and reloads a special round for it - that just is not me. With that rant noted, finding the right ammo that your gun likes can make a sane man nuts. I have seen some expensive guns and barrels that will only shoot premium tier ammo well and everything else printed like a shotgun and sometimes just one particular offering of those premium match rounds.
If you have the patience and record keeping diligence to work through a variety of ammo or build up and test your rifle’s favorite, that process can demonstrate a pretty dramatic swing of results that can take your rifle from zero to hero. In most cases, I have found that testing a cross section of match grade ammo of various bullet weights will identify a clear winner of bullet weight and velocity to target finding a less expensive or handloads that closely matches that spec. Oddly enough, I have found that some barrels can favor a rather inexpensive factory ammo over a match grade option. I have a few rifles that prefer Hornady Frontier ammo to match options as an example. Assuming you have your build done right, ammo can help tighten up the groups even more.
Barrel Recommendations - 2021 is a crazy time to buy anything, but if you can buy a factory headspaced bolt from a quality barrel manufacturer like JP, White Oak Armament, Ballistic Advantage/Aero, and Faxon, you will be miles ahead of any headspacing problems.
Most charge a nominal fee of around $50 for the service, but it is worth it. My barrel recommendations would also be in that order. Black Hole is another great barrel option, however they do not offer factory headspace bolts and I do not have much experience with them since they started Columbia River barrels. For the super high end, look to Lilja, Hart, Criterion, Lothar, and Bartlein, but request factory headspaced bolts with the barrels.
AccuWedge - Though the AccuWedge does nothing more than give you a solid lockup between the upper and lower, it will increase accuracy just with a more solid platform that does not move around. For those that are not running higher end AR receivers with integrated tension screws, this is a great option that only costs about $10 per half dozen rubber AccuWedges. They can be trimmed if needed. A more solid platform will net improved accuracy and this simple little accessory removes the wiggle between your receivers.
Torque Matters - A Fat Wrench and a good ½” drive mechanical torque wrench can be indispensable for delivering consistent torque. Equal torque on all your scope rings and base mounts, barrel nuts, and even handguard screws can deliver improvements that stack up and also prevent costly damage. I would place a fair wager that most accuracy improvements/degradation and overall parts damage to ARs during the build process is due to torque. If the receiver is not lapped, it introduces barrel torque that makes shots wander, improper optics torque can definitely impact accuracy, and even over-tightening a muzzle break has shown to impact accuracy. Of note, muzzle brakes should be secured with red lock-tite and achieve the correct position with finger tightening (no wrench). This tip from one of my pro-friends has increased accuracy for me as the barrel heats. Whether you are using a crush washer or fixed indexing washer, there should be no torque between the muzzle brake device and the end of the barrel.
Comfort - Comfort is usually not discussed much, however it should be after all the AR15 platform can be the most comfortable platform in the world to shoot from. If you do not comfortably drop in behind your rifle on the bench or the rifle does not point naturally in the field, it will undoubtedly impact speed and accuracy. All too often I see shooters on the range contorting themselves in painful looking positions just to shoot their rifle. Oftentimes that means scope rings that are too low/high, cheek rests too low/high, optics that are mounted too close/far from the eye box, stocks that do not fit the shooter, and rifle supports that do not allow for a comfortable position. A great book Long-Range Precision Rifle - Expanded Edition talks about this in great detail that your accuracy will be impacted if you are contorting yourself behind the rifle.
FINAL THOUGHTS
For many, all the above seems like a nit-picky level of detail that can make eyes roll, but as I have learned and been humbled, all these things do make a huge difference. Even if you have a bone stock budget AR and want to get the most from it, working through these tips will likely deliver large noticeable accuracy improvements. If you are into AR15 building and into building a quality AR15 at home, then you should consider the Brownells AR15 Upper Receiver Lapping Tool along with tools like a good vise, punches, calibrated torque wrench, LockTite thread lock and retaining compound, copper anti-seize, headspace gauges, and Geissele Action Rod or some type of action block are all valuable tools to get the most from your platform.
SPECS
Brownells AR15 Upper Receiver Lapping Tool - $34.99
Brownells Garnett Lapping Compound - $19.99
Fat Wrench
Torque Wrench
Headspacing Gauges/Gages Go, No-Go, Field
SOURCES
Brownells - http://www.brownells.com